The links between poor sleep and symptoms of dementia have been studied for years, but there's still much we don't know about how lack of sleep may contribute to cognitive decline in conditions like Alzheimer's disease.

One of the limitations in our data is many observational studies looking at sleep duration and dementia have relatively short follow-up windows, or only examine senior individuals at their outset – whereas the development and manifestation of dementia symptoms often take place over decades, and can start emerging much earlier in people's lives.

To provide a longer-term analysis examining how sleep in the years leading up to old age might affect dementia outcomes, a research team led by first author and epidemiologist Séverine Sabia from the University of Paris looked at data from the Whitehall II cohort, an ongoing longitudinal study of the health of over 10,000 British civil servants.

The Whitehall II study began in 1985, and now offers a dataset spanning over three decades, giving us a comprehensive body of evidence to examine whether and how sleep duration and quality in mid-life and onwards might be linked with subsequent dementia diagnoses later in life.

According to the researchers, the link does exist in the Whitehall data, and it's something to be aware of.

"Persistent short sleep duration at age 50, 60, and 70 compared to persistent normal sleep duration was … associated with a 30 percent increased dementia risk independently of sociodemographic, behavioral, cardiometabolic, and mental health factors," the authors write in their paper.

It's worth clarifying that studies like this are only observational, meaning the link discovered is only a correlation between short sleep and increased risk – not a causative mechanism.

In other words, the researchers aren't saying poor sleep causes dementia; only that many of the people in the Whitehall cohort who later developed dementia tended to sleep less than other participants who were less likely overall to develop the condition.

In the study, normal sleep duration was defined to mean seven hours of sleep per night, with long sleep meaning eight hours or longer nightly.

Short sleep duration was six or less hours of sleep each night, and participants who consistently only got that amount of sleep at night showed a higher risk of developing dementia at all ages.

People who slept normal amounts showed the least incidence of dementia, and there was no clear evidence of a link between longer-than-normal sleep duration and dementia (although some other studies have found such an association too).

While the study relies heavily on self-reported data from the participants – and the Whitehall cohort aren't necessarily reflective of other populations in society – the findings do help to strengthen our knowledge on the ties between poor sleep and dementia, even though the causative mechanisms themselves remain enigmatic, and much further research is warranted.

One of the difficulties, as the researchers acknowledge, is evidence of a bidirectional association between sleep dysfunction and the pathophysiological changes in dementia.

In other words, abnormal sleep doesn't just predict dementia; the development of early dementia might also affect people's sleep, in which case poor sleep could be considered an early symptom of the disease.

Of course, just because the link is bidirectional doesn't mean either direction is mutually exclusive or has importance. Until more is definitively known, we have to consider all the possibilities the evidence suggests, experts say.

"As dementia results from changes in the brain, it is unsurprising that people with dementia often have disrupted sleep patterns," says psychiatrist and dementia researcher Tom Dening from the University of Nottingham in the UK, who was not involved with the study.

"Maybe it is simply a very early sign of the dementia that is to come, but it's also quite likely that poor sleep is not good for the brain and leaves it vulnerable to neurodegenerative conditions like Alzheimer's disease."

As it stands, there are still many unknowns in this area of dementia research, but even as the science evolves, it's never too late to implement other lifestyle changes linked to improve chances of maintaining healthy brain function as we get older.

"While there is no sure-fire way to prevent dementia, there are things within our control that can reduce our risk," says Sara Imarisio, Head of Research at Alzheimer's Research UK, who wasn't involved with the study.

"The best evidence suggests that not smoking, only drinking in moderation, staying mentally and physically active, eating a balanced diet, and keeping cholesterol and blood pressure levels in check can all help to keep our brains healthy as we age."

The findings are reported in Nature Communications.